In many electrical converter devices, e.g. in a frequency converter, there is a need for a filter appliance between a converter stage arranged to produce, e.g. by Pulse Width Modulation (PWM), multiphase alternating voltage and an external electrical system, e.g. an electrical motor or generator, that is electrically connected to the electrical converter device. The filter appliance can be needed, for example, for suppressing common-mode electrical current, for reducing slew rate of output voltages (du/dt) of the converter stage, for over-current protection, for reducing radio frequency emissions, and/or for suppressing harmonics of voltages and/or of electrical currents. The filter appliance may comprise one or more coils preferably made of foil conductors that are suitable for high frequency electrical currents, especially, when the thickness of the foil conductors is less than the penetration depth of signals on the frequency range under consideration.
FIG. 1a shows a filter appliance according to the prior art and suitable for a three-phase electrical converter device. FIG. 1b shows a section view taken along the line A-A shown in FIG. 1a. The filter appliance comprises three first electrical terminals 101, 102 and 103 for receiving alternating voltages produced by a multiphase electrical converter device and three second electrical terminals 104, 105 and 106 for electrically connecting to an external electrical system, e.g. to an electrical motor or generator. The filter appliance comprises three coils 107, 108 and 109. Each of the coils is formed of foil conductor and is electrically connected between one of the first electrical terminals and one of the second electrical terminals. A detail of the coil 107 is shown in a partial magnification 110. The coil 107 comprises a foil conductor 111. The successive turns of the coil 107 are spaced apart from each other with the aid of an insulating layer 112. It is also possible that, instead of the insulating layer 112, there are discrete spacer-elements arranged to keep the successive turns of the coil 107 apart from each other. The foil conductor can be made of, for example, copper or aluminium. The insulating layer 112, or correspondingly the spacer-elements, may comprise for example mica insulator. The filter appliance may further comprise a magnetic core structure 113 made of magnetically amplifying material, i.e. material having the relative permeability greater than unity (μr>1). The magnetic core structure can be made of ferromagnetic or paramagnetic material. The magnetic core structure is preferably made of soft magnetic material that provides low hysteresis and eddy current losses, e.g. electrical steel sheets, soft magnetic powder, ferrites, etc. It is also possible that the filter appliance is an air-core filter i.e. there is no magnetically amplifying material on routes of magnetic fluxes produced by electrical currents flowing in the coils 107, 108 and 109.
An inconvenience related to the above-described filter appliance is caused by the capacitive coupling between successive turns of the coils. The capacitive coupling between successive turns of the coils causes worsening of the high frequency properties of the filter appliance.